“It appears to be retroactive,” said Olson, director of the council’s health program.

Dicamba has been around for decades, but its use was limited because the chemical evaporates easily, particularly at higher temperatures, and drifts, damaging plants in fields that have not been sprayed with the chemical.

In 1974, another pesticide, Roundup, containing glyphosate, was introduced and became one of the world’s most widely used pesticides. That overuse led to weeds like pigweed becoming resistant.

Three chemical companies, Monsanto,DowDuPont and BASF, developed what they claimed were “low-volatility” versions of dicamba that didn’t evaporate so easily. Monsanto genetically altered soybeans and cotton to better tolerate dicamba, an approach it had also used for Roundup.

Monsanto claims the damage was because farmers didn’t follow directions on the pesticide label in spraying. The company sued Arkansas regulators, claiming the ban was based on “unsubstantiated theories regarding product volatility that are contradicted by science.”

Local restrictions on pesticides were upheld in a 1991 unanimous Supreme Court decision.

The case, Wisconsin Public Intervenor v. Mortier, was about Casey, Wis., a town of 404 in rural northwest Wisconsin that required a permit to apply pesticides on public lands, aerial spraying of private lands, and private land that could have public uses. Landowner Ralph Mortier challenged the town’s 1985 ordinance.